A fiber-optical sensor of this kind which comprises two lightwave conductors (or optical fiber) having two adjacently situated portions between which light can be coupled is known from DE-PS No. 34 15 242. In this patent specification a lightwave conductor consists of a light-conducting core and a cladding surrounding this core. A part of the cladding is removed from the adjacently situated portions of the lightwave conductors. When a liquid is present at this gap, a part of the light is transferred from one lightwave conductor to the other. Light is coupled into one lightwave conductor by a light transmitter device. The light coupled into the other lightwave conductor via the gap and the light remaining in the lightwave conductor are received and evaluated in a light receiver device. All interference (for example, attenuation) occurring between the light transmitter device and the gap is eliminated by forming the ratio of the two light components. Subject to the condition that the lightwave conductors are subject to the same effects between the gap and the light receiver device, the interference induced thereby is also eliminated. This condition, however, can only be rarely satisfied in reality. This fiber optical sensor is capable of measuring, for example temperatures of a temperature-sensitive liquid introduced into the gap. Moreover, pressure measurement is possible where the refractive index of the liquid changes as a function of the pressure. In that case an additional, pressure-resistant and complex device must be provided which introduces and keeps the liquid in the gap.
Furthermore, from DE-OS No. 30 12 328 which corresponds substantially to U.S. Pat. No. 4,342,919 issued Aug. 3, 1982 there is known a fiber-optical measuring device in which a lightwave conductor and a light-conductive or light-absorbing synthetic material are adjacently arranged. The light conductor comprises a light-conductive core from which the cladding has been removed. Under the influence of the hydrostatic pressure in a container filled with liquid, the synthetic material is pressed against the lightwave conductor over a given distance which corresponds to the level in the container. Thus, a given part of the light coupled into the lightwave conductor is coupled out to the synthetic material. Via an intermediate interference filter, the lightwave conductor is connected, using further lightwave conductors, to two sources (light transmitter device) of light of different wavelength and to a light receiver device. The light from the first light source is reflected by the interference filter and reaches, via a branch of the lightwave conductors, the light receiver device as a reference signal. The two light sources are alternately powered, the corresponding electrical signals being detected in synchronism therewith in order to obtain a measurement signal by the formation of a quotient in a divider circuit, the magnitude of said signal depending on how much light has been coupled out by pressing the synthetic material against the stationary lightwave conductor and how much light is conducted to the light receiver device, via the lightwave conductors, after reflection. In this measuring apparatus the attenuation effect of the lightwave conductors between the interference filter and the light transmitter device and the light receiver device is eliminated by formation of the quotient. Drift characteristics of the branches in the transmitter device and the receiver device, however, are not eliminated. Because a part of the light is coupled into the synthetic material, only a part of the transmitted light is used for evaluation of the measurement. This may lead to inaccurate measurement results notably in the case of high liquid levels, where a very large part of the light is coupled out.